/*
 * Copyright (C) 2009 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.base;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;

/**
 * An object with an operational state, plus asynchronous {@link #start()} and
 * {@link #stop()} lifecycle methods to transfer into and out of this state.
 * Example services include webservers, RPC servers and timers. The normal
 * lifecycle of a service is:
 * <ul>
 * <li>{@link State#NEW} -&gt;</li>
 * <li>{@link State#STARTING} -&gt;</li>
 * <li>{@link State#RUNNING} -&gt;</li>
 * <li>{@link State#STOPPING} -&gt;</li>
 * <li>{@link State#TERMINATED}</li>
 * </ul>
 * 
 * If the service fails while starting, running or stopping, its state will be
 * {@link State#FAILED}, and its behavior is undefined. Such a service cannot be
 * started nor stopped.
 * 
 * <p>
 * Implementors of this interface are strongly encouraged to extend
 * {@link com.google.common.util.concurrent.AbstractService} or
 * {@link com.google.common.util.concurrent.AbstractExecutionThreadService},
 * which make the threading and state management easier.
 * 
 * @author Jesse Wilson
 * @since 2009.09.15 <b>tentative</b>
 */
public interface Service {
    /**
     * If the service state is {@link State#NEW}, this initiates service startup
     * and returns immediately. If the service has already been started, this
     * method returns immediately without taking action. A stopped service may
     * not be restarted.
     * 
     * @return a future for the startup result, regardless of whether this call
     *         initiated startup. Calling {@link Future#get} will block until
     *         the service has finished starting, and returns one of
     *         {@link State#RUNNING}, {@link State#STOPPING} or
     *         {@link State#TERMINATED}. If the service fails to start,
     *         {@link Future#get} will throw an {@link ExecutionException}, and
     *         the service's state will be {@link State#FAILED}. If it has
     *         already finished starting, {@link Future#get} returns
     *         immediately. Cancelling the returned future is unsupported and
     *         always returns {@code false}.
     */
    Future<State> start();

    /**
     * Initiates service startup (if necessary), returning once the service has
     * finished starting. Unlike calling {@code start().get()}, this method
     * throws no checked exceptions.
     * 
     * @throws InterruptedRuntimeException
     *             if the thread was interrupted while waiting for the service
     *             to finish starting up.
     * @throws RuntimeException
     *             if startup failed
     * @return the state of the service when startup finished.
     */
    State startAndWait();

    /**
     * Returns {@code true} if this service is {@link State#RUNNING running}.
     */
    boolean isRunning();

    /**
     * Returns the lifecycle state of the service.
     */
    State state();

    /**
     * If the service is {@link State#STARTING} or {@link State#RUNNING}, this
     * initiates service shutdown and returns immediately. If this is
     * {@link State#NEW}, it is {@link State#TERMINATED terminated} without
     * having been started nor stopped. If the service has already been stopped,
     * this method returns immediately without taking action.
     * 
     * @return a future for the shutdown result, regardless of whether this call
     *         initiated shutdown. Calling {@link Future#get} will block until
     *         the service has finished shutting down, and either returns
     *         {@link State#TERMINATED} or throws an {@link ExecutionException}.
     *         If it has already finished stopping, {@link Future#get} returns
     *         immediately. Cancelling this future is unsupported and always
     *         returns {@code false}.
     */
    Future<State> stop();

    /**
     * Initiates service shutdown (if necessary), returning once the service has
     * finished stopping. If this is {@link State#STARTING}, startup will be
     * cancelled. If this is {@link State#NEW}, it is {@link State#TERMINATED
     * terminated} without having been started nor stopped. Unlike calling
     * {@code stop().get()}, this method throws no checked exceptions.
     * 
     * @throws InterruptedRuntimeException
     *             if the thread was interrupted while waiting for the service
     *             to finish shutting down.
     * @throws RuntimeException
     *             if shutdown failed
     * @return the state of the service when shutdown finished.
     */
    State stopAndWait();

    /**
     * The lifecycle states of a service.
     */
    public enum State {

        /**
         * A service in this state is inactive. It does minimal work and
         * consumes minimal resources.
         */
        NEW,

        /**
         * A service in this state is transitioning to {@link #RUNNING}.
         */
        STARTING,

        /**
         * A service in this state is operational.
         */
        RUNNING,

        /**
         * A service in this state is transitioning to {@link #TERMINATED}.
         */
        STOPPING,

        /**
         * A service in this state has completed execution normally. It does
         * minimal work and consumes minimal resources.
         */
        TERMINATED,

        /**
         * A service in this state has encountered a problem and may not be
         * operational. It cannot be started nor stopped.
         */
        FAILED
    }
}
